WASHINGTON – Nobody thought the bill sponsored by Republican Rob Portman of Ohio and Democrat Jeanne Shaheen of New Hampshire was anything more controversial than a modest effort to encourage energy savings in new homes and older commercial buildings.

Yet despite having more than enough votes to pass, the bill collapsed in May because of a lethal blend of Republican delaying tactics and Democrats’ reluctance to cast unpopular votes. By evening’s end, a discouraged Sen. Mary Landrieu, D-La., complained, “It’s a shame that all the hard work that’s gone into this basically ended in a draw.”

It was yet another example of why the Senate seems so paralyzed. On the same floor where legislators once approved epic laws guaranteeing equal rights for African-Americans and cleaning the nation’s air, today’s Senate features acrimonious debates that often involve such opaque phrases as filibuster, cloture, unanimous consent and fill the tree.

This is more than politics as usual. The Senate’s inability to pass meaningful legislation is reaching historic levels. It’s not just controversial bills, such as the plan to build the Keystone XL pipeline from Canada, that get buried. Among the bills the Senate failed to approve was an extension for another year of about $85 billion in tax breaks, including the $250 deduction for teachers who foot the bill for classroom expenses, and mortgage debt forgiveness for people with negative equity in their homes.

‘D.C. is so messed up’

“I’ll be frank with you,” Portman said. “When I go home and talk about this issue … there’s not a whole lot of interest in this because people’s sense is D.C. is so messed up.”

Each political party accuses the other of firing the first shot. “Republicans are filibustering on steroids,” said James Manley, a former aide to Senate Majority Leader Harry Reid, D-Nev. By contrast, Walt Riker, a onetime adviser to former Senate Majority Leader Bob Dole, R-Kan., complained “this version of the Democratic Party is the most partisan political force in my lifetime.”

But a quarter-century of slippery steps to the bottom by both political parties has led to the current impasse, prompting Sarah Binder, a senior fellow at the Brookings Institution in Washington to say “there is no innocent party.”

Not only are Republicans and Democrats divided ideologically between conservatives and liberals, but the comity that once existed — Dole and Sen. Robert Byrd, D-W.Va., made a point of shaking hands at the end of every session — seems a quaint relic from another era.

Just last year, as Reid pushed to end the practice of using a filibuster to kill a Cabinet nomination, Senate Minority Leader Mitch McConnell, R-Ky., tartly warned that if Reid went through with his plan “our friend the majority leader is going to be remembered as the worst leader ever.”

McConnell’s scorching rhetoric has been matched by Reid, a pugnacious former boxer who last year assailed Portman, accusing him of “pontificating” about an amendment to the immigration bill and complaining, “We are spending all this time because he has been aggrieved in some way.”

Riker said of the rancor frequently on display: “The kind of name-calling I see on the Senate floor is astounding.”

Instead of legislating, both parties tend to shadow box. As majority leader, Reid can place any bill on the floor. Republicans often respond by threatening to delay or kill the measure by offering scores of amendments, many which have nothing to do with the bill.

Reid responds by using his unique authority to prevent Republicans from offering amendments. It is a parliamentary maneuver called “filling the tree.”

How the maneuver works

Here’s how it works: Reid introduces a series of amendments. Next he offers second-degree amendments that propose minor changes in his original amendments. By “filling the tree’ with his amendments, as allowed under Senate rules, Reid blocks Republicans from gaining floor votes on their amendments.

Finally he files for what is called a cloture vote, a maneuver designed to end Republican filibusters, or delaying tactics. By doing so he forces Republicans to choose: Vote for cloture, which ends floor debate and allows lawmakers to vote on the bill, or vote against cloture, which essentially kills the bill.

In the past, filling the tree was rarely used. When Dole filled the tree in the 1990s, Riker had to hunt down the Senate parliamentarian to ask what it meant. Reid has used the tactic 85 times in the past four years. By comparison, the previous six majority leaders used it 40 times.

WWII veteran and former Senate Majority Leader Bob Dole waves after taking part in a wreath laying ceremony at a 10th anniversary ceremony for the WWII Memorial in Washington, Saturday, May 24, 2014. (AP Photo/Molly Riley)

FILE - In this Oct. 3, 2013, file photo Sen. Mary Landrieu, D-La., speaks on Capitol Hill in Washington. Deciding which party controls the U.S. Senate for the final two years of President Barack Obamaís tenure could come down to women _ both the handful who are running in significant races and the moderate female voters who often tip close elections. (AP Photo/ Evan Vucci, File)